Combine like terms. These are the terms with the same variables raised to the same exponents. In this expression, $\displaystyle{8}{{a}}^{{2}}{{b}}^{{3}}\usepackage{cmbright}\Huge\$ and $\displaystyle{3}{{a}}^{{2}}{{b}}^{{3}}\usepackage{cmbright}\Huge\$ are like terms and are combined by adding their coefficients.

$\displaystyle{8}{{a}}^{{2}}{{b}}^{{3}}+{9}{{a}}^{{3}}{{b}}^{{2}}+{3}{{a}}^{{2}}{{b}}^{{3}}={8}{{a}}^{{2}}{{b}}^{{3}}+{3}{{a}}^{{2}}{{b}}^{{3}}+{9}{{a}}^{{3}}{{b}}^{{2}}={\left({8}+{3}\right)}{{a}}^{{2}}{{b}}^{{3}}+{9}{{a}}^{{3}}{{b}}^{{2}}={11}{{a}}^{{2}}{{b}}^{{3}}+{9}{{a}}^{{3}}{{b}}^{{2}}\usepackage{cmbright}\Huge\$